Move eager loading of hasMany associations to FetchRequest

[groue]

This PR fixes two things:

• Eager loading of hasMany associations (Eager Loading of HasMany associations #525) was defined on QueryInterfaceRequest, not on the FetchRequest protocol. This had GRDB 4.0 define many overloads, for requests of rows or records, just to activate this feature. Such overloads were necessary in RxGRDB as well.
• The result of the fundamental FetchRequest.prepare method was a tuple, and impossible to extend.

Now FetchRequest is able of eager loading, even if this isn't visible on the public API (one can't add eager loading to custom FetchRequest):

+struct PreparedRequest {
+    var statement: SelectStatement
+    var adapter: RowAdapter?
+    init(statement: SelectStatement, adapter: RowAdapter? = nil)
+}
 
 protocol FetchRequest {
+    // deprecated
     func prepare(_ db: Database, forSingleResult singleResult: Bool) throws -> (SelectStatement, RowAdapter?)
+    func preparedRequest(_ db: Database, forSingleResult singleResult: Bool) throws -> PreparedRequest
 }